1. Field of Invention
The present invention relates to a fuel pump apparatus for an internal combustion engine, and more particularly to a fuel pump installed in a fuel tank for pumping up and feeding fuel to a carburetor or fuel injection nozzles mounted on an intake manifold of the engine.
2. Brief Description Prior Arts
In a field of the fuel pump of the above kind, various types of fuel pumps have been known and practically used, such as a roller pump, a centrifugal pump, an axial flow type pump, a regenerative type pump and so on. The regenerative type pump having a closed vane impeller is recently more often used since it produces a high discharge pressure (2 to 3 kg/cm.sup.2) with less noise and pulsated discharge pressure than roller pumps.
However, since the fuel in a fluid passage of the regenerative type pump is agitated by a plurality of vanes of the impeller, fuel vapor and/or cavitation is easily produced. Particularly when the regenerative type pump is used in a severe condition, such as at a high temperature and/or at a low ambient pressure, the fuel vapor is produced and retained in a fuel passage, causing a so-called vapor-lock with a result that the pump can no longer pump up and feed the fuel although the impeller is rotated.
In order to avoid such a vapor lock, it is known in the art, for example in U.S. Pat. No. 3,418,991, to provide a small vapor discharge port in a pump housing for communicating a pump chamber with the outside of the pump chamber, so that a small amount of fuel flows out of the pump chamber through the port along with the fuel vapor.
According to the observations of the present inventors, a satisfactory result for avoiding the vapor-lock cannot be obtained when the small vapor discharge port is simply formed in the pump housing and particularly when the pump is operated under the above-described severe condition.